LARVAL TREMATODES OF THE NORTHUMBERLAND COAST. 449 



violently shaking its tail, and then contracting and elongating 

 its body. The body is covered with small backwardly directed 

 Spines for about two-thirds of its length from the neck down- 

 wards. In one specimen of Paludestrina this cercaria was 

 found encysted all among the rediae and tailed cercariae, but 

 it is also to be found encysted in the foot of the little bivalve 

 Sc?'ob2Cielarm tenuis (Mont.). This is a small white shell 

 living amongst weed and sand in the flats uncovered by the 

 tide at low water. It is very abundant on Fenham Flats, and 

 occurs with Paludestrina stagnalis. The cysts are o"i2 mm. 

 across {E. seetindum o*2i-o'25 mm.), with a thin wall of two 

 layers, and are perfectly transparent, showing the larva curled 

 up inside. This encysted cercaria agrees almost exactly with 

 that described by Villot as Cej'caria leptosoma, the larva of 

 Distoma leptosoma which he found in the adult state in the 

 Dunlin Tringa alpina. He found this encysted in the foot of 

 Scrobicularia tenuis^, measuring o'lio to o"i20 mm. across, 

 pharynx 0*020 by o"oo6 mm., oral sucker much smaller than 

 ventral, length of head spine 0-020 mm., agreeing fairly well 

 with my measurements except in the breadth of the pharynx, 

 which is greater in the present specimens. Villot has followed 

 the development of this worm from Scrobicularia tenuis to the 

 intestine of the Dunlin and up to its adult stage. The ventral 

 sucker in his figures of the adult worm is much higher up 

 than in my larval forms, and the pharynx narrower. The 

 ■position of the ventral sucker in Villot's encysted Trematodes 

 is not referred to by him, only a very brief description being 

 given. I believe my specimen to be Distcwiu?n ( Jl chinos to?num) 

 leptosomum, Creplin, and if this be the case, the life history of 

 this Trematode is complete : — ; 



Distoinuni ( Echinostomum) leptosomuni, Creplin. 



Final Host. 

 Dunlin Tringa alpina. 



First Host. 

 Fahidestrina stapnalis. 



Second Host. 



Scrolnailaria tenuis 



or encysts in 



Paludestrina stagnalis. 



* Villot, Ann. des Sc. Nat., Zool.. 6 s&-., Tom. 8, 



